This has never worked for the combination of Sauer + Linux, but you can use the following bind which has the same effect:bind "U" [ fullscreen (! $fullscreen) ]

I see texture not found everywhere, WTF went wrong with the install?

Chances are you use OSX and something went wrong when you tried to merge the data and packages folder. Make sure you don’t use the folder where the Sauerbraten files are located but the special configuration folder, as described in the install.md. When you have already some files inside the configuration folder, try copy stuff by hand, while making sure that you chose merge in the finder dialogue.

I'm on whindows and I'm sure i followed every step from the install.md. But when I double click on the Sauerbraten shortcut it starts vanilla Sauerbraten

You have multiple Sauerbraten installs on your system. Search for sauerbraten.exe, identify the correct directory and update your shortcut.

/comed isn't working for me, what did I do wrong?

You obviously haven’t read the install.md. You forgot to copy the data/comed-gui.cfg into /data/ (/comed-gui.cfg in release 0.1)